(1) Field of the Invention
The present invention relates to an improved system and method for automatically controlling a vehicle speed to a desired cruising speed.
(2) Background of the art
A Japanese Patent Application First Publication (Kokai) sho 47-35692 published on Nov. 25, 1972 exemplifies one of such automatic cruising speed controlling systems.
In the above-identified Japanese Patent Application Publication, target vehicle speed setting means is provided for setting a target vehicle speed according to a vehicle driver's intention and vehicle speed detecting means is provided for measuring the vehicle speed. In addition, controlling means is provided for calculating an integral error .epsilon..sub.I (=Vs-V) of the detected vehicle speed V with respect to the above-described target vehicle speed Vs and proportional error .epsilon..sub.p (=V'-V) between the present vehicle speed V and vehicle speed V' before one control period for a predetermined period (e.g., 300msec.) and calculating a valve opening duration t.sub.p (=K.sub.I x.epsilon..sub.I +K.sub.p x.epsilon..sub.p) by adding these errors .epsilon..sub.I, .epsilon..sub.p which are multiplied by gain constants K.sub.I and K.sub.p corresponding to these errors, respectively. An acceleration valve for driving an engine throttle valve to open is operated for the duration calculated as t.sub.p, if the value of t.sub.p is positive. When the value of t.sub.p indicates negative, a deceleration valve for driving the engine throttle valve to close is operated for the duration calculated as t.sub.p. Consequently, the values .epsilon..sub.I and .epsilon..sub.p are controlled to approach zero so that the vehicle speed V matches the target vehicle speed Vs.
A throttle actuator for driving the throttle valve according to each open duration of such acceleration and deceleration valves includes a pneumatic diaphragm, one end of which is connected via an accelerator link to the engine throttle valve and receives an actuating force due to pressure difference between internal air pressure and atmospheric pressure which occurs due to the opening and closing operations of the acceleration and deceleration valves. The accelerator link is linked to the throttle valve via an accelerator wire. Thus, an opening angle of the throttle valve is controlled through the valve open duration t.sub.p described above. Consequently, an engine output torque T.sub.E according to the opening angle of the throttle valve is generated and converted into a vehicle driving torque T.sub.p through a transmission and tires in a power train. Then, the vehicle runs at the vehicle speed V due to the driving torque T.sub.p.
However, in the above-described automatic cruising speed controlling system a manipulated variable (valve opening duration) is calculated and outputted using the unique and same control equation (t.sub.p =K.sub.I x.epsilon..sub.I +K.sub.p x.epsilon..sub.p) although a transfer function of the throttle actuator and accelerator link is largely dependent upon a variation of frictional forces at the time when the accelerator link for every vehicle strokes, a variation of responsive characteristic for every actuator, temperature change and aging effects, it o is difficult to maintain a cruising speed controlling characteristic constant at all times and for each vehicle. In addition, vehicle occupants receive shocks due to the acceleration and deceleration of the vehicle and feel uncomfortable.